diff --git a/tornado/gen.py b/tornado/gen.py
index 62bc144b..7ca9afb5 100644
--- a/tornado/gen.py
+++ b/tornado/gen.py
@@ -1249,7 +1249,9 @@ def convert_yielded(yielded):
     .. versionadded:: 4.1
     """
     # Lists and dicts containing YieldPoints were handled earlier.
-    if isinstance(yielded, (list, dict)):
+    if yielded is None:
+        return moment
+    elif isinstance(yielded, (list, dict)):
         return multi(yielded)
     elif is_future(yielded):
         return yielded